Ionization chambers are commonly used for detecting x-ray photons and other ionizing radiation. X-ray photons will interact with atoms of a heavy detector gas to produce electron-ion pairs. The x-ray photons are, generally, absorbed by a gas atom which emits a photoelectron from one of its electronic levels. The photoelectrons move through the gas, interacting with and ionizing other gas atoms, to produce a shower of electrons and positive ions which may be collected on suitable electrodes to produce an electric current flow. If such electron-ion pairs are produced in a region between two electrodes of opposite polarity, they will drift along electric field lines to the electrodes and will yield an electric current. The electric current flow between the electrodes is a function of the total number of x-ray photons interacting in the vicinity of those electrodes.
The probability of detection of an x-ray photon is a function of the atomic weight of the gas and of the number of gas atoms lying between the collector electrode. Thus, high sensitivity detectors may be constructed from a gas of high atomic weight at a relatively high pressure. Detector sensitivity may also be increased by increasing the spacing, and therefore the number of gas molecules, between the electrodes. Increased electrode spacing, however, increases the distance which the electron-ion pairs drift for collection and thus tends to increase the recovery time of the detector. An increased electric field gradient between the electrodes will tend to increase the ion drift velocity and thus somewhat shorten the recovery time of the detector. However, one is limited in the electric gradient increase which it is feasible to use, since avalanche gas gain will begin to occur, causing gain uncertainty and, eventually, gas breakdown. Also increasing detector voltage causes undesirable increases in detector microphonic sensitivity.
Arrays of ionization chambers are typically used to measure x-ray intensity distributions in computerized transverse axial tomography equipment. In a typical application of such equipment, a moving x-ray source is repeatedly pulsed to transmit x-ray energy along a plurality of distinct ray paths through a body undergoing examination. Energy transmitted through the body is detected in an ionization chamber array and interpreted, by use of a digital computer, to produce x-ray images of internal body structures. My copending patent application with Nathan R. Whetten, Ser. No. 616,930, filed Sept. 26, 1975, describes an array of ionization chambers which may be effectively utilized in computerized transverse axial tomography equipment. That disclosure is incorporated by reference herein, as background material.
The data collection rate in computerized tomography equipment incorporating ionzation chamber detector arrays is limited by the recovery time of the individual detector cells. The time between x-ray pulses must be sufficiently long to allow collection of substantially all of the charged particles within the detector cells.
The electrons produced in ionization chambers are known to drift very rapidly to the anode while the positive ions move much more slowly to the cathode. In general, the electron current cannot, however, be independently measured in prior art ionization chambers since it is masked by a displacement current which is generated in the anode circuit by the positive ions flowing away from the anode.
There is, however, one exception to the preceding statement. A simple two-electrode ionization chamber can detect independently the electron component if the x-ray pulse is very short as compared to the ion drift time. In that case, the electron component stands out as an intense short pulse above the slowly-changing ion displacement current. However, in most computerized tomography x-ray equipment, it is not feasible to achieve a sufficient x-ray flux level if the x-ray pulse is short in comparison to the ion drift time even at the maximum current now achievable in conventional x-ray tubes. Instead, in present-day computerized tomography systems, it is necessary to use an x-ray pulse which is comparable in length to the ion drift time (typically a few milliseconds). In such case, there is no way to separately measure the electron current component in prior-art ionization chambers.
Such prior art ionization chambers are described, for example, in Ionization Chambers and Counters Experimental Techniques, B. B. Rossi and H. H. Staub, McGraw-Hill 1949, at Chapter 5 which text is incorporated herein as background material.
Mechanical vibrations which may be transmitted to the electrodes of prior art ionization chambers vary the electrode spacing and capacitance and thus tend to introduce microphonic error currents into the detector circuit. The electrical noise produced by these microphonic currents may necessitate the use of an increased radiation exposure in order to produce tomographic images of a given resolution.